The present invention relates to a tire changing machine.
Tire changing machines of the type known as "American-style" machines have long been known--see patent U.S. Pat. No. 4,335,772 (Bubik et al.)--comprising a bed on which a fixed platform is provided bearing a pair of mutually opposite conical portions on which the wheel rim of tire wheels having diameters included in a wide range of values is located, and thus such tire changing machines can be used for wheels of various sizes. From the top platform of the bed there extend upwards a fixed, hollow and externally threaded shaft, a rotatable shaft coaxially arranged inside the fixed hollow shaft and terminating with a flat top portion projecting from the hollow shaft, and a locking pin which is designed to engage a wheel located on the fixed platform and is parallel to the fixed shaft.
A tire bead breaking operation is carried out by the squashing action of two mutually opposite shoes: one of which is supported by a lateral column carried by the bed and arranged to act by means of a lever system actuated by a suitable pneumatic cylinder from above downwards on the upward-facing bead of a tire, whereas the other is located within the bed and arranged to act upwards by means of a lever system which is actuated by a suitable pneumatic cylinder.
Once a local bead breaking is obtained, the operator then completes bead breaking along the whole periphery of the wheel by forcing the tire to rotate by hand. Should this require too much effort, the operator must release the heel, turn it through 180.degree. about its own axis, lock it again and repeat the operation. He can then remove the tire by inserting a suitable manual tire tool in the flat portion of the rotating shaft.
Apart from the physical effort by the operator, such tire changing machines have bead breaking shoes, which must slidingly engage with the wheel rim and therefore cannot be used with light alloy wheel rims, since they would be damaged, and are provided with two pneumatic cylinders which are designed to actuate the upper and the lower bead breaking shoes, respectively. This entails the need of requiring considerable power for actuating the bead breaking shoes.
Another drawback of such conventional tire changing machines is the simultaneous movement of the bead breaking shoes and the rotating shaft, since the same power source, namely the pneumatic cylinders, is used to actuate both the shoes and the rotating shaft with no possibility to exclude neither the former nor the latter. Accordingly, during a tire mounting operation the lower shoe acts in a direction opposite to the tire mounting direction and thus hinders the tire mounting. Moreover, since the rotating shaft is driven by a pneumatic cylinder, it can perform an angular movement which is limited to less than 360.degree. and a return stroke is required which entails a waste of time.
Moreover, to avoid sudden accelerations of the tire tool caused by this kind of driving system, when stress is no longer applied to the tire tool, hydraulic speed limiting circuits had to be provided for the pneumatic cylinders. This results in considerable shortcomings in the manufacture and difficulties in use.
In addition, tire changing machines of this kind can be used only with American-type wheels, that is to say, wheels which have a plurality of through holes in one of which the locking pin can be engaged, and not with other kind of wheels, most of which according to more recent trends do not possess such holes, e.g. spoke wheels.